1. Technical Field
The present disclosure relates to a replacement necessity determination device for a snap-in valve to which an air pressure detection device is integrally coupled.
2. Description of the Related Art
There is known a valve unit, which is used for a wheel assembly of a vehicle, e.g., a motor vehicle, and has a function of detecting an air pressure. The valve unit includes a valve for adjusting a tire air pressure, and an air pressure detection device integrally coupled to one end of the valve. The air pressure detection device includes an air pressure detection sensor and an acceleration sensor for detecting the rotation of the wheel assembly, and the acceleration sensor is configured to detect an acceleration of the wheel assembly in a radial direction thereof. The air pressure detection device of the valve unit is arranged in an air chamber formed by a rim portion and the tire held by the rim portion, and the valve of the valve unit is mounted to the rim portion under a state in which the valve is inserted into a hole formed in the rim portion of a wheel.
When the mounting of the valve to the rim portion is defective, the air at a high pressure inside the tire leaks via a gap between the valve and the rim portion, resulting in a fast decrease in the tire air pressure. Moreover, the center of gravity of the valve unit exists on the air pressure detection device side with respect to a mounting portion of the valve to the rim portion. Thus, when the mounting of the valve to the rim portion is defective, the valve unit is pivoted by a centrifugal force caused by the rotation of the wheel assembly, and the acceleration sensor cannot appropriately detect the acceleration of the wheel assembly in the radial direction thereof. In order to prevent this problem from occurring in the valve unit, for example, in Japanese Patent Application Laid-open No. 2005-170224, there is described an abnormality detection device for detecting the defective mounting of the valve unit to the rim portion.
In the abnormality detection device described in Japanese Patent Application Laid-open No. 2005-170224, a displacement amount of the pivot of the valve unit is predicted based on a ratio of a second acceleration calculated through differentiation of a wheel assembly speed detected by a wheel assembly speed sensor to a first acceleration detected by the acceleration sensor. When the ratio of the second acceleration to the first acceleration increases, and the displacement amount of the pivot of the valve unit exceeds a reference value, the mounting of the valve to the rim portion is determined to be defective. This abnormality detection device is configured to detect the defective state through use of the fact that as the displacement amount of the pivot of the valve unit increases, an angle of an acceleration detection direction of the acceleration sensor with respect to the radial direction of the wheel assembly increases and a detection value of the acceleration sensor thus becomes less than an original value.
The valve used for the wheel assembly of the vehicle, e.g., a motor vehicle, is classified into a clamp-in valve and a snap-in valve. The clamp-in valve is fixed to the rim portion by using a packing to clamp the rim portion and tightening a nut on the air chamber side. In contrast, the snap-in valve includes a valve stem having a tube shape and an elastic body covering the valve stem, and is mounted to the rim portion under a state in which the elastic body is compressed between the valve stem and the rim portion.
The abnormality detection device described in Japanese Patent Application Laid-open No. 2005-170224 is a device for detecting abnormality of mounting, which is a state in which the valve is inclined, and abnormality of a loosened nut in the clamp-in valve. Even when the abnormality occurs to the clamp-in valve, a displacement amount of pivot of the valve unit does not increase so much. Thus, the ratio of the second acceleration to the first acceleration is a relatively small value when the mounting of the valve is determined to be defective.
When the valve of the valve unit is the snap-in valve, and the valve unit pivots as a result of a centrifugal force caused by the rotation of the wheel assembly, the elastic body is deformed, resulting in a secular degradation. Thus, it is conceivable to apply the abnormality detection device described in Japanese Patent Application Laid-open No. 2005-170224 to the detection of the secular degradation of the valve unit including the snap-in valve, thereby determining a life of the snap-in valve caused by the secular degradation of the elastic body.
However, the snap-in valve is mounted to the rim portion by a compression force of the elastic body, and hence the angle of the pivot of the snap-in valve generated by the application of the centrifugal force is more than that in the clamp-in valve, and increases as the acceleration of the wheel assembly increases and the centrifugal force thus increases. Further, as the deformation of the elastic body is repeated by the pivot of the valve, and the secular degradation of the elastic body progresses, the elastic modulus of the elastic body gradually decreases, and the pivot angle of the clamp-in valve gradually decreases for the same centrifugal force.
Thus, the ratio of the second acceleration to the first acceleration increases as the acceleration of the wheel assembly increases and the centrifugal force thus increases, but the amount of the increase gradually decreases as the secular degradation of the elastic body progresses. Thus, the ratio of the second acceleration to the first acceleration cannot be used as an indication of the secular degradation of the elastic body, and hence the abnormality detection device described in Japanese Patent Application Laid-open No. 2005-170224 cannot be used to determine whether or not the replacement of the snap-in valve caused by the secular degradation of the elastic body is necessary.